1. Field of the Invention
The present invention relates to accelerator technique, and more particularly to apparatus for electron beam irradiation of objects.
2. Prior Art
When objects are irradiated by charged particles, including electrons, for example, in apparatus for radiation and chemical treatment of materials it is required to provide an irradiation field of considerable extension equal at least to the width of an irradiated object. The whole surface of the object is exposed to irradiation by displacing of the object lengthwise across the irradiation field.
Besides, the irradiation field should be uniform to provide predetermined properties of the irradiated material equal, over the whole surface of irradiation, i.e. it is required to obtain uniform distribution of energy of the charged particles over the surface of the irradiated object to provide equal depth of penetration of the charged particles into the material of the object.
Known in the art are apparatus for electron beam irradiation of objects, wherein shaping of extended irradiation fields is based on scanning of an electron beam, i.e. on displacement of the beam of small cross-sectional area over the irradiated surface by means of its deflection by a time-modulated field, most frequently by a magnetic field. In the apparatus of this type the maximum permissible width of the material to be irradiated depends on the vertical dimension of the vacuum chamber of the apparatus. Thus, for example, in order to sweep the electron beam for 1 meter the vertical dimension of the vacuum chamber should be about 2 meters and further increase in the width of the irradiated objects considerably increases the vertical dimension of the apparatus. If the amount of deflection of an electron beam is increased while maintaining the same height of the vacuum chamber, nonuniformity of the irradiation of the objects over their width occurs due to the fact that the angle of incidence of electrons onto the objects at the extreme positions of the beam will be substantially different from the right angle corresponding to the electron trajectory at the central beam position.
Known in the art is an apparatus for electron beam irradiation of objects (Cf. FRG Application No. 2,901,056 published 1979), comprising an electron beam shaper, a deflecting electromagnet with a frame-type magnetic circuit to direct the electron beam to the irradiated object substantially at an angle of 90.degree., and a vacuum chamber to transport the electron beam from the shaper through the magnetic circuit and further through an exit window of the vacuum chamber onto the surface of the irradiated object, the deflecting magnet being located wherever necessary either outside the vacuum chamber embracing the latter, or inside the vacuum chamber. The electromagnet has a number of windings arranged at its poles and geometrically displaced relative to one another along the poles. The electromagnet windings are connected in turn to a supply source through a commutator, whereby the field of the electromagnet moves in the direction of the line equidistant to the surface of the irradiated object.
The apparatus according to the abovementioned FRG Application eliminates the drawbacks inherent in the apparatus, using the scanning of an electron beam, i.e. it can provide a uniform irradiation field of practically any desirable extension without increase in the height of the apparatus owing to horizontal arrangement of the electron beam shaper and the vacuum chamber. However, operation of the deflecting magnet under alternating field conditions results in the following complications in the apparatus design:
use of laminated magnetic circuit in the deflecting electromagnet;
use of a special commutation circuit for connecting the electromagnet windings to the supply source, provided with a commutator control circuit;
when the deflecting magnet is arranged outside the vacuum chamber the latter should either have sufficiently thin walls (0.3-0.5 mm) of stainless steel, said walls being obligatory corrugated like belows to provide its mechanical strength, or it should be made of dielectric such as, for example, ceramics;
when the deflecting magnet is arranged inside the vacuum chamber it is necessary to keep the low level of gas release within the volume of the vacuum chamber from the laminated magnetic circuit of the electromagnet and its windings, this being achieved by baking said assemblies in epoxy or other low gassing compounds with mineral fillers.